As the most ancient of lubricants—said to extend back into the later Stone Age of man—grease is nonetheless perhaps the least understood and yet most widely used form of lubrication. This situation is not for want of effort to more clearly understand its response to operating conditions but more because of the wide range of those operating conditions and the range of components that over the millennia have been found to make it more usable.
A property having a major effect on the utility of grease is its resistance to oxidation and the consequences of this resistance on its ability to maintain an effective level of lubrication. Such resistance to oxidation and degradation under the stresses imposed by high temperatures and thousands of hours of operation has always been an important feature of grease performance, and a particular focus of its use and development over the years. As a consequence, increasingly advanced bench techniques have been developed to improve the understanding of both how the oxidation resistance can be enhanced by formulation and how this resistance can be lost in service.
Thus, oxidation stability is a very important property of lubricating greases. And so, any bench test capable of measuring this property is important to the degree that it is capable of predicting either the shelf-life or the service-life of a grease, or both.
Prior Art Grease Oxidation Test
In the 1940's the desire for a readily applicable bench test for quality control of a grease formulation by measuring its oxidation resistance led to the development and publication of ASTM Test Method D942 in 1947—a test method which became a standard and continues to be applied. Ref., ASTM Method of Test D942-02, “Oxidative Stability of Lubricating Greases by the Oxygen Pressure Vessel Method” (Reapproved 2007), ASTM Vol. 5, pp. 352-356, 2010; IP Method of Test 142/85 (92). It is routinely used to measure batch-to-batch grease oxidation stability, and also has been used in screening new grease formulations in development. That test requires exposure of 20 grams of a grease in five glass dishes with an exposed surface area of approximately 25 cm2 per dish, thus 125 cm2 per test. The dishes are stacked with an about 5-mm gap between them. Compare, FIG. 1. The combined stack of grease-filled dishes are then inserted into a cylindrical pressure chamber and exposed to oxygen of not less than 99.5% purity at an initial pressure of 100 pounds per square inch (PSI=690 kPa) and room temperature, which is then increased to 99±0.5° C. Under this increased temperature, the oxygen pressure is carefully released to maintain no more than 110±2 PSI. The test is typically continued for a chosen period of 100 or 200 hours, with resultant decrease in oxygen pressure as a result of grease oxidation taken as the test result.
Some Limitations of ASTM D942 as known in the Prior Art
Despite being relatively simple and straightforward, it is clearly stated in the ASTM D942 method that the test is severely limited in application to any other use than quality control of a grease formulation. For example, it is stated that the test should not be applied to compare the oxidation stability of the tested grease to actual behavior in service. Similarly, because of differences among grease formulations and the effects of oil volatility, comparison of oxidation resistances of different greases for a given application is also not recommended. Nor should the test be used to predict the stability of grease stored in containers or the oxidation stability of greases used in bearings and motors since only controlled oxidation of a physically stable surface of grease is used in the test.
Moreover, the ASTM D942 Test Method as known in the prior art is a very time consuming (100˜500 hours) and labor intensive test requiring comparatively large sized samples. Thus, it is inapplicable to measuring small sample sizes taken from operating bearings or other applications to determine remaining oxidation resistance.
On the other hand, its simplicity is desirable. Perhaps an improved test may preserve this.